The present invention is directed to new and useful multilayer heat shrinkable film formulations. One distinguishing feature of a shrink film is the film's ability, upon exposure to a certain temperature, to shrink or, if restrained from shrinking, to generate shrink tension within the film
The manufacture of shrink films, as is well known in the art, may be generally accomplished by extrusion (single layer films) or coextrusion (multilayer films) of thermoplastic resinous materials which have been heated to their flow or melting point from an extrusion or coextrusion die in, for example, either tubular or planar (sheet) form. After a post extrusion quenching to cool by, for example, the well-known cascading water method, the relatively thick "tape" extrudate is then reheated to a temperature within its orientation temperature range and stretched to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials will vary with the different resinous polymers and/or blends thereof which comprise the material. However, the orientation temperature range for a given thermoplastic material may generally be stated to be below the crystalline melting point of the material but above the second order transition temperature (sometimes referred to as the glass transition point) thereof. Within this temperature range it is easy to effectively orient the material.
The terms "orientation" or "oriented" are used herein to generally describe the process step and resultant product characteristics obtained by stretching and immediately cooling a resinous thermoplastic polymeric material which has been heated to a temperature within its orientation temperature range so as to revise the inter-molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838-81. When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is simultaneously applied in two directions biaxial orientation results. The term oriented is also herein used interchangeably with the term "heat shrinkable" with these terms designating a material which has been stretched and set by cooling while substantially retaining its stretched dimensions. An oriented (i.e. heat shrinkable) material will tend to return to its original unstretched (unextended) dimensions when heated to an appropriate elevated temperature.
Returning to the basic process for manufacturing the film as discussed above, it can be seen that the film, once extruded (or coextruded if it is a multilayer film) and initially cooled to by, for example, cascading water quenching, is then reheated to within its orientation temperature range and oriented by stretching. The stretching to orient may be accomplished in many ways such as, for example, by "blown bubble" techniques or "tenter framing". These processes are well known to those in the art and refer to orientation procedures whereby the material is stretched in the cross or transverse direction (TD) and/or in the longitudinal or machine direction (MD). After being stretched, the film is quickly quenched while substantially retaining its stretched dimensions to rapidly cool the film and thus set or lock-in the oriented molecular configuration.
Of course, if a film having little or no orientation is desired, e.g. non-oriented or non-heat shrinkable film, the film may be formed from a non-orientable material or, if formed from an orientable material may be "hot blown". In forming a hot blown film the film is not cooled immediately after extrusion or coextrusion but rather is first stretched shortly after extrusion while the film is still at an elevated temperature above the orientation temperature range of the material. Thereafter, the film is cooled, by well-known methods. Those of skill in the art are well familiar with this process and the fact that the resulting film has substantially unoriented characteristics. Other methods for forming unoriented films are well known. Exemplary, is the method of cast extrusion or cast coextrusion which, likewise, is well known to those in the art.
After setting the stretch-oriented molecular configuration the film may then be stored in rolls and utilized to tightly package a wide variety of items. In this regard, the product to be packaged may first be enclosed in the heat shrinkable material by heat sealing the shrink film to itself where necessary and appropriate to form a pouch or bag and then inserting the product therein. If the material was manufactured by "blown bubble" techniques the material may still be in tubular form or it may have been slit and opened up to form a sheet of film material. Alternatively, a sheet of the material may be utilized to over-wrap the product. These packaging methods are all well known to those of skill in the art. Thereafter, the enclosed product may be subjected to elevated temperatures by, for example, passing the enclosed product through a hot air or hot water tunnel. This causes the enclosing film to shrink around the product to produce a tight wrapping that closely conforms to the contour of the product. As stated above, the film sheet or tube may be formed into bags or pouches and thereafter utilized to package a product. In this case, if the film has been formed as a tube it may be preferable to first slit the tubular film to form a film sheet and thereafter form the sheet into bags or pouches. Such bag or pouch forming methods, likewise, are well known to those of skill in the art.
The above general outline for manufacturing of films is not meant to be all inclusive since such processes are well known to those in the art. For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of these patents are generally representative of such processes.
Alternative methods of producing films of this type are known to those in the art. One well-known alternative is the method of forming a multilayer film by an extrusion coating rather than by an extrusion or coextrusion process as was discussed above. In extrusion coating a first tubular layer is extruded and thereafter an additional layer or layers is sequentially coated onto the outer surface of the first tubular layer or a successive layer. Exemplary of this method is U.S. Pat. No. 3,741,253. This patent is generally representative of an extrusion coating process.
Many other process variations for forming films are well known to those in the art. For example, multiple layers may be first coextruded with additional layers thereafter being extrusion coated thereon. Alternatively, two multilayer tubes may be coextruded with one of the tubes thereafter being extrusion coated or laminated onto the other. The extrusion coating method of film formation is preferable to coextruding the entire film when it is desired to subject one or more layers of the film to a treatment which may be harmful to one or more of the other layers. Exemplary of such a situation is a case where it is desired to irradiate one or more layers of a film containing an oxygen barrier layer comprised of one or more copolymers of vinylidene chloride and vinyl chloride. Those of skill in the art generally recognize that irradiation is generally harmful to such oxygen barrier layer compositions. Accordingly, by means of extrusion coating, one may first extrude or coextrude a first layer or layers, subject that layer or layers to irradiation and thereafter extrusion coat the oxygen barrier layer and, for that matter, other layers sequentially onto the outer surface of the extruded previously irradiated tube. This sequence allows for the irradiation cross-linking of the first layer or layers without subjecting the oxygen barrier layer to the harmful effects thereof.
Irradiation of an entire film or a layer or layers thereof may be desired so as to improve the film's resistance to abuse and/or puncture and other physical characteristics. It is generally well known in the art that irradiation of certain film materials results in the cross-linking of the polymeric molecular chains contained therein and that such action generally results in a material having improved abuse resistance. When irradiation is employed to accomplish the cross-linking, it may be accomplished by the use of high energy electrons, ultra violet radiation, X-rays, gamma rays, beta particles, etc. Preferably, electrons are employed up to about 20 megarads (MR) dosage level. The irradiation source can be any electron beam generator operating in a range of about 150 kilovolts to about 6 megavolts with a power output capable of supplying the desired dosage. The voltage can be adjusted to appropriate levels which may be for example 1,000,000 or 2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Many apparatus for irradiating films are known to those of skill in the art. The irradiation is usually carried out at a dosage up to about 20 MR, typically between about 1 MR and about 20 MR, with a preferred dosage range of about 2 MR to about 12 MR. Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, 0.degree. C. to 60.degree. C. may be employed.
Cross-linking may also be accomplished chemically through utilization of peroxides as is well known to those of skill in the art. A general discussion of cross-linking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Figers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a Library of Congress Catalog Card Number of 64-22188.
Another possible processing variation is the application of a fine mist of a silicone or anti-fog spray to the interior of the freshly extruded tubular material to improve the further processability of the tubular material.
The polyolefin family of shrink films and, in particular, the polyethylene family of shrink films provide a wide range of physical and performance characteristics such as, for example, shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in linear dimension in a specified direction that a material undergoes when subjected to elevated temperatures while unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to tear apart), heat sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and tear resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), elongation (the degree the film will stretch or elongate at room temperature), elastic memory (the degree a film will return to its original unstretched (unelongated) dimension after having been elongated at room temperature), and dimensional stability (the ability of the film to retain its original dimensions under different types of storage conditions). Film characteristics play an important role in the selection of a particular film and they differ for each type of packaging application and for each type of package. Consideration must be given to the product size, weight, shape, rigidity, number of product components and other packaging materials which may be utilized along with the film material and the type of packaging equipment available.
In view of the many above-discussed physical characteristics which are associated with polyolefin films and films containing a polyolefin constituent and in further view of the numerous applications with which these films have already been associated and those to which they may be applied in the future, it is readily discernable that the need for ever improving any or all of the above described physical characteristics or combinations thereof in these films is great, and, naturally, ongoing. In particular, the quest for a heat shrinkable polyethyne film having an improved combination of elongation, elastic memory, puncture resistance, tear propagation resistance and heat sealability has been ongoing since such a film could compete well in the trayed product (for example, meat such as poultry parts) overwrap market. Historically, polyvinyl chloride (PVC) films have been utilized in this overwrap application because of their good degree of elongation and elastic memory. PVC was superior to conventional heat shrinkable films with regard to overwrapped trayed products which were subject to moisture loss because the PVC was elastic and continued to contract as the product lost moisture and shrank during the distribution cycle. The result was a tight package which was somewhat unattractive because it was leaky. The elasticity of PVC also allowed automatic overwrapping machinery to stretch the PVC material about the trayed product during overwrapping of the product and the associated tray. In spite of the fact that the package was leaky, PVC proved superior to conventional heat shrink packages because such conventional packaging materials possessed relatively poor elasticity or elastic memory. Thus, when a product wrapped in such a material shrank from moisture loss during the distribution cycle the film did not also shrink and the result was a loose package having a shopworn appearance.
Unfortunately, PVC has several drawbacks associated therewith which those in the art wish to improve upon or wholly eliminate. Exemplary of these drawbacks is the fact that PVC tray overwrap film generally evidences both (1) poor seal integrity and (2) poor abuse resistance.
The poor seal integrity of PVC overwrap films arises at least in part from the fact that the PVC material in PVC overwrapped trays is tack welded to itself as opposed to being hermetically sealed. Thus, the liquid purge or juices which exude from the overwrapped trayed meat products will leak through a tack sealed PVC overwrapped tray and result in a package that is unsightly and messy from a consumer viewpoint. This drawback appears to be irreconcilably linked to PVC since attempts to hermetically seal PVC in a commerial tray overwrap environment usually result in "burn-through" of the PVC material.
Another major drawback of PVC tray overwrap material is, as stated above, the material's poor resistance to abuse. In this regard the PVC material tends to tear along the edges of the overwrapped tray if rubbed during transit by another tray or an enclosing carton.
U.S. Pat. No. 4,617,241 discloses a very useful stretch/shrink film with excellent elongation and heat shrinkability properties. In some applications, it is desirable to use such a film but with higher oxygen transmission properties. Certain vegetables and other food products, such as mushrooms, exhibit longer shelf life if a "breathable", i.e. high oxygen transmission rate film is employed as the packaging material.